Many personal care products currently available to consumers are directed primarily to improving the health and/or appearance of skin and/or hair. For example, there are a variety of topical skin care products available that are directed to delaying, minimizing, or even eliminating skin wrinkling and other histological changes typically associated with the aging of skin or environmental damage to human skin. For at least some people, fine lines and wrinkles in the skin are a reminder of the disappearance of youth. As a result, the elimination of wrinkles has become a booming business in youth-conscious societies. Treatments range from cosmetic creams and moisturizers to various forms of cosmetic surgery.
It is not uncommon for personal care products such as skin lotions and creams to include vitamins, vitamin derivatives, or other active ingredients for improving skin appearance. For example, vitamin A, also referred to as retinol, is known for use in topical skin care compositions to provide skin health and/or appearance benefits. Vitamin A, along with its derivatives, form a class of compounds commonly referred to as “retinoids.” At one time, retinoids were primarily used for the treatment of acne. More recently, retinoids have also been used in the treatment of photo- and/or intrinsically-aged skin. While retinoids may provide desirable skin health and/or appearance benefits, some retinoids such as retinol and retinoic acid have been associated with skin irritation. As a result, retinyl esters such as retinyl acetate and retinyl palmitate, which have been shown to be milder on skin compared to retinols and retinoic acid, are sometimes included in skin care compositions. However, these milder forms of retinoids may not provide the same benefit(s) (e.g., exfoliation) or level of benefit as retinol or retinoic acid.
Retinoids can also be undesirably reactive and susceptible to degradation, leading to a shorter than desired product shelf life (e.g., product yellowing or acidification). Some known sources of degradation include oxidation, light exposure (e.g., ultraviolet radiation), and heat (e.g., temperatures of 40° C. or more). Thus, additional processing steps and/or packaging constraints may be needed to minimize degradation of the retinoid in a personal care composition. For example, careful processing in an oxygen-free environment, use of an oxygen impermeable packaging, and/or the inclusion of one or more anti-oxidants and/or chelating agents added to the composition may at least partially alleviate the amount and/or rate of retinoid degradation. However, special packaging and processing steps are not always practical or economical. And even when implemented, such steps may not be enough to suitably alleviate retinoid degradation issues.
When formulating personal care products containing retinoids, the product should be aesthetically pleasing, deliver active ingredients as intended, and exhibit suitable stability. In some instances, such retinoid containing compositions are in the form of an oil-in-water (“O/W”) emulsion system, wherein the retinoid is carried primarily within the oil phase and is protected from oxidation by an oil-soluble antioxidant. See, e.g., U.S. Pat. Nos. 3,906,108; 4,466,805; and 4,247,547. When compared to water-in-oil (W/O) emulsions, O/W emulsions are sometimes thought to be less greasy feeling, more compatible with other such emulsion products, less occlusive, easier to remove from the skin, more aesthetically pleasing, and/or more economical to manufacture. However, it has been shown that retinyl esters in such compositions quickly lose their activity and either oxidize or isomerize to non-efficacious chemical forms. Thus, the amount of retinoid actually available to provide a beneficial effect is reduced, in an unacceptably short period of time, to an ineffective quantity and eventually to only trace quantities. See, WO93/00085.
To overcome the lack of retinoid stability in an O/W emulsion system, some manufacturers turned to a water-in-oil emulsion system, for example, as described in U.S. Pat. Nos. 4,826,828 and 4,720,353; European Patent Nos. EP 0,343,444 and EP 0,330,496. However, the use of W/O emulsions, with or without additional oil soluble antioxidants, still fail provide suitable retinoid stability.
Accordingly, it would be desirable to provide a stable, retinoid-containing skin care composition in the form of an oil-in-water emulsion. It would also be desirable to provide a stable, retinoid-containing skin care composition that does not irritate skin.